Two primary uses for boilers in residential buildings include providing domestic hot water (DHW) and providing hot water for indoor heat. Typical boilers can do this in several ways. Two of these ways include, a boiler and an indirect water configuration, and a boiler with tankless heater configuration. Normally, the indirect water heater has DHW storage tank built in to it.
In the boiler and indirect water heater system, the closed boiler and piping system is initially filled with cold water from a water source, such as municipal water supply or well water. The boiler heats the water, and outputs hot water. The hot water output of a boiler is configured in two circuits. A pump or automatic valve(s) are employed to divert hot water from the boiler to either circuit. Space heating is accomplished by flowing hot water through a loop in the circuit which includes a radiator or other device for transferring heat out of the hot water and into air.
If the controller calls for more DHW, the hot water from a boiler is diverted to an indirect water heater to heat up the municipal water. The cooled down boiler water flows back to boiler. The DHW is stored in the tank until it can be used for various domestic hot water uses such as showers, laundry, dishwashers, and any other residential or commercial need for hot water. This type of system requires a lot of room because the boiler, the circuit, and the storage tank must be stored.
Another option includes producing hot boiler water for indoor heat and also DHW. A typical boiler will include a heat exchanger including a coil which may be made of thin copper tubing rolled into a compact circular shape. It is inserted into a chamber in the boiler where it is surrounded by water. The water surrounding the copper tubing is referred to as boiler water or system water. Cold water from a water source such as municipal or well water is drawn through the coil. The water flowing through the coil is often used for DHW.
A heat source, such as hot gases generated by burning fuel, or an electronic heat source applies heat to the boiler water. The boiler water then transfers heat to the DHW. In a typical boiler, with a tankless heater, a relatively large amount of boiler water surrounds the copper coil, and as heat is transferred to the DHW from the boiler water, the boiler water cools. The cooling effect generates a natural current in the boiler water which permits cool boiler water to flow away from the copper coil and hot boiler water to flow toward the copper coil. The natural current is an important factor in efficiently heating the DHW. A relatively large reservoir of boiler water is required to produce the natural current. Typical dimensions for a boiler of this type which can make about 3 gallons per minute of domestic hot water are 22 inches wide, 40 inches high, and 39 inches deep.
A second characteristic many conventional boilers with tankless heaters have is a heavy weight. By having a heavy boiler and large volume of boiler water, a large thermal mass sustains the heating for the DHW. As the DHW is heated, heat is removed from the boiler and the boiler water. If the boiler and boiler water cool too much as the DHW is heated, the heat transfer to the DHW looses efficiency and is hampered. A boiler that can make about 3 gallons per minute of DHW requires a typical boiler to weigh about 460 lbs.
There are some applications that require not only both space heating and DHW but also require compact or lightweight boiler. For example, installation space that was available for a boiler may only be about 22 inches wide, 28 inches high and 27 inches deep and require the boiler to generate about 3 gallons per minute of domestic hot water. Therefore, a small compact boiler is desired that can generate a similar amount of domestic hot water as larger and heavier boilers.